1. Field of the Invention
This invention relates to a process of producing an aluminum substrate for making magnetic recording media, such substrate having an anodically oxidized film or layer on the surface thereof. More particularly, the invention relates to a process of producing an aluminum substrate for making high-density magnetic recording media, such substrate having substantially no black spot defect and having excellent smoothness and heat resistance.
2. Description of the Prior Arts
Recently, there has been much demand for increased recording density of magnetic recording media, such as magnetic disk, etc. For responding to such demand, it is necessary to reduce thickness of the magnetic medium layer to be formed on a surface of the substrate and the spacing between the magnetic head and the magnetic medium. The substrate of such magnetic recording media must exhibit superior surface properties with respect to smoothness, and hardness.
As a substrate for making high-density magnetic recording media as above-mentioned, an aluminum substrate having an anodic oxide film on its surface has been used.
Aluminum substrate having such an anodic oxide film is desirable because the anodic oxide film formed on the surface of aluminum is hard and excellent in wear resistance, has good polishability, whereby a smooth surface in high accuracy can be easily obtained, and a thin magnetic layer can easily formed on its surface.
Hitherto, for easily forming an anodic oxide film having suitable hardness for making magnetic recording media on an aluminum substrate, commonly an electrolytic treatment using a sulfuric acid solution has been performed as disclosed in the British Pat. No. 1,493,160 of Nov. 23, 1977. However, the aluminum substrate having formed thereon an anodic oxide film using a sulfuric acid solution (hereinafter, is referred to as a sulfuric acid anodic oxide film) has the faults as described hereinafter and these faults are handicaps to increasing the recording density of a magnetic recording media.
One of these faults is a so-called black spot defect. Impurities such as iron, silicon, etc., present in aluminum or an aluminum alloy crystallize as intermetallic compounds, which exist at the surface of aluminum or the alluminum alloy as microscopic spots. Such microscopic spots do not form anodic film properly during electrolytic treatment. The spots are, at the beginning, very fine, i.e. of sub-micron order, but become larger with the growth of the anodic oxide film and show pit-like fine defects of 5 to 10 .mu.m in diameter on a sulfuric acid anodic oxide film having a thickness greater than 5 .mu.m or more. If a substrate has many of these defects, the signal error in the magnetic recording medium is large and hence the existence of the defect is undesirable.
Another one of the faults is thermal cracking. In the case of an aluminum substrate for high-density magnetic recording medium, it is required to apply .alpha.-Fe.sub.2 O.sub.3 to the surface of the aluminum substrate by sputtering or vapor deposition, followed by heating to 300.degree.-400.degree. C. for forming .gamma.-Fe.sub.2 O.sub.3 but when the aluminum substrate is heated to such a high temperature, the sulfuric acid anodic film formed on the substrate is cracked, whereby inferior products are liable to result. Accordingly, the thickness of the anodic oxide film must be reduced to as thin as 1 to 3 .mu.m, which results in reducing the head crash resistance of the aluminium substrate and hence the occurence of cracking is undesirable.
As the result of investigation on improving an aluminum substrate having anodic oxide film, the inventors have discovered that when an aqueous chromatic acid solution is used for the formation of the anodic oxide film on an aluminum substrate and electrolysis is performed at a constant voltage method higher than the voltage employed for conventional chromic acid electrolysis, an aluminum substrate free of black spot defects and thermal cracking can be produced.
Also, it has been discovered that when forming the anodic oxide film over 10 .mu.m in thickness on an aluminum using the foregoing chromic acid process, a hardness of thus formed film becomes about or over 300 Hv in Vickers hardness, which is required for head crush resistance and scratch resistance in this kind of aluminum substrate.